In conventional photographic techniques, color images are usually obtained by the reaction of oxidation products of aromatic primary amine color developing agents, which are formed in the redox reaction of such agents and optically exposed silver halide, with dye-forming compounds known as couplers. Upon the reaction of a coupler with the oxidation product of a color developing agent, the coupler is coupled to the oxidized color developing agent at its reactive site called a coupling position to form a dye. Dyes formed in the coupling reaction are indoaniline dyes, azomethine dyes, indamine dyes or indophenol dyes, depending upon compositions of the couplers and the developing agents used. In a multicolor photographic element, the subtractive color process is generally employed for formation of color images, and the dyes formed therein upon coupling are, in general, cyan, magenta, and yellow dyes, produced in silver halide emulsion layers having sensitivities in the red, green, and blue regions of the spectrum, respectively. Alternatively, dyes may be formed in the layers adjacent to the silver halide emulsion layers. For the purpose of reduction of processing time, color photographic materials have increasingly been processed at temperatures higher than normal temperature (20.degree.-25.degree. C.), and processing at a temperature of 30.degree. C. or above is very typical nowadays. Hydrophilic colloids, e.g., gelatin in which silver halides, couplers, and other additives are dispersed, are softened and swelled in the high temperature rapid processing. Consequently, the emulsion films are exposed to the peril of being damaged, or to peeling off of the support. Therefore, the emulsion layers must be hardened to withstand the high temperature development. The hardening is effected by passing the emulsion layers through a hardening solution initially in the development processing, or by adding a prehardener such as formaldehyde, dialdehyde, mucochloric acid or the like to silver halide emulsions at the time of preparation.
It is of prime importance to the hardening of photographic layers that the highest degree of hardness is achieved as quick as possible after drying, whereby a stable degree permeability of a developing solution into emulsion layers is ensured.
Recently, it has acquired a greater importance to use a hardener capable of acting rapidly for photographic purposes, instead of using one which exerts a hardening effect over many hours. The rapidity of the hardening reaction makes it feasible to avoid changes in properties of photographic materials during storage and continuous drop in permeability of a developing solution into photographic materials.
The expression "rapidly acting hardeners" is intended to include compounds which can cause cross-linking of gelatin in a very short time, if possible, during coating and/or drying. They must complete the cross-linking reaction within 24 hours.
Rapidly acting hardeners of interest include carbodiimides as described in U.S. Pat. Nos. 2,938,892 and 3,098,693, E. Schmidt, F. Hitzler and E. Lahde, Ber., Vol. 71, 1933 (1938), and, Bull. Soc. Chim. France, 1360 (1956); dihydroquinone compounds described in German Patent Application (OLS) No. 2,322,317; carbamoylpyridinium compounds described in German Patent Application (OLS) Nos. 2,225,230, 2,317,677 and 2,439,551; and carbamoyloxypyridinium compounds described in German Patent Application (OLS) No. 2,408,814.
One of the characteristics common to the above-described rapidly acting hardeners is that they can activate a carboxyl group. This function can be illustrated by the known reaction of carbodiimide with carboxylic acid. In this reaction, an N-acylurea or an acid anhydride is employed as the active group. In the case of proteins which have both carboxyl and amino group, the reaction further proceeds to result in the formation of peptide linkages between an activated carboxyl group and an amino group. Therefore, compounds of such a kind are known as peptide reagent (Chemical Review, Vol. 67, pp. 107-152 (1967)).
However, the compounds of the above-described kind retard development in a magenta color-forming layer. Accordingly, utilization of these compounds as rapidly acting hardeners in color photographic materials has been limited.